There are many different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays. Because CCD technology provides large array sizes having small pixel sizes (high resolution), they are well-suited for imaging applications where space is at a premium. However, in some applications, CCD imagers are susceptible to radiation damage, require good light shielding to avoid image smear, exhibit relatively high power dissipation, and are difficult to integrate with CMOS processing. As a matter of contrast, a complementary metal-oxide semiconductor (CMOS) imager may exhibit low voltage operation and low power consumption, while providing relatively low fabrication costs and compatibility with existing CMOS control logic and image processing. Further, CMOS imagers may also offer random access to the image data without using pixel to pixel transfer operations during image acquisition.
A CMOS imaging device includes an array of pixel cells, each of which may comprise a photosensor, such as a photogate, photoconductor, or photodiode overlying a substrate. The photosensor is used to accumulate charge as a function of received light over an integration period. Each pixel cell may further comprise a readout circuit that includes an output field effect transistor (FET) formed in the substrate with a sensing node, such as a floating diffusion node, connected to the gate of an output transistor. The imager may include an additional transistor for transferring charge from the underlying portion of the substrate to the sensing node, and a transistor for resetting the sensing node to a specified charge level prior to charge transfer.
Image blur from a moving subject during the image acquisition time (e.g., exposure time) may be resolved by either tracking the imaged subject with the CMOS imager, or reading the CMOS image sensor at higher speeds. Reading image sensors at higher speeds often translates to using higher speed processing, with a commensurate increase in capacity to store multiple frames of the digitized image in external memory. Thus, there is a need for apparatus, systems, and methods that operate to increase the speed at which image data can be processed within and by image sensors.